1. Field of the Invention
The present invention relates to a rear projection screen provided with at least a Fresnel lens sheet and a lenticular lens sheet.
2. Description of the Related Art
The outline constitution of an ordinary rear projection screen is graphically shown in FIG. 1, in which 1 indicates a Fresnel lens sheet, and 2 indicates a lenticular lens sheet. In a rear projection screen, in general, a Fresnel lens sheet and a lenticular lens sheet are kept in contact with each other. Rays of image light having been projected on the rear of a rear projection screen provided with a Fresnel lens sheet and a lenticular lens sheet are collimated or converged by the Fresnel lens sheet, and the convergent rays shall have a focal point on the side of viewers spaced from the screen by from 5 to 20 meters. The collimated rays or convergent rays having passed through the Fresnel lens sheet are diffused by the lenticular lens sheet to a great extent in the horizontal direction, thereby enabling viewers to see the images in a broad field of view in the horizontal direction. To enlarge the viewing field of the images not only in the horizontal direction but also in the vertical direction, the lenticular lens sheet is generally made of a material that contains a diffusing agent.
FIG. 2 is a cross-sectional view graphically showing the outline constitution of a Fresnel lens sheet. In general, the Fresnel lens sheet is a sheet with concentric Fresnel lenses formed on its light-outgoing surface at a fine constant pitch, as illustrated. Optionally for intended capabilities, a fine lenticular lens sheet having a function of diffusing rays of image light in the vertical direction may be provided adjacent to the light-incoming surface of the Fresnel lens sheet. For preventing rays of image light from reflecting inside the Fresnel lens sheet to give ghost light, the Fresnel lens sheet may contain a diffusing agent. FIG. 3 is a cross-sectional view graphically showing the outline constitution of a lenticular lens sheet. As in FIG. 3, the lenticular lens sheet is a sheet with semicylindrical lenticular lenses formed on both the light-incoming surface and the light-outgoing surface each at a constant pitch. In general, black stripes are provided between the adjacent semicylindrical lenticular lenses on the light-outgoing surface of the sheet. These are for preventing external light from reflecting on the sheet to hit viewers. Apart from the illustrated one, known is a lenticular lens sheet of a different type having semicylindrical lenses formed only on the light-incoming surface.
Where a Fresnel lens sheet is combined with a lenticular lens sheet for use in image displays, the lenses formed on the two sheets each at a constant pitch shall have an area through which light could pass and an area through which light could not pass, thereby giving moire fringes. Precisely, as in FIG. 2, the region "a" of each Fresnel lens could pass light to the viewer's side but the region "b" thereof could not, whereby the light having passed through the Fresnel lens sheet gives concentric fringes at a fine constant pitch. On the other hand, a part of the lenticular lens could pass light, while the other thereof could not, with the result that the light having passed through the sheet gives vertical fringes at a fine constant pitch, as in FIG. 3.
In the combination of the two sheets, it is difficult to completely prevent such moire fringes, but it is possible to reduce them to a negligible level for practical use. Some techniques have been developed for reducing moire fringes to such a degree that they do not trouble viewers. For this, the lens pitches of the two sheets to be combined are optimized by specifically defining the ratio of the lens pitch of the Fresnel lens sheet to that of the lenticular lens sheet. For example, the ratio of the lens pitch of lenticular lenses to that of Fresnel lenses is defined to fall between N+0.35 and N+0.45 (N indicates a natural number) (see Japanese Patent Laid-Open No. 95525/1984); the ratio of the lens pitch of lenticular lenses to that of Fresnel lenses is defined to fall between N+0.25 and N+0.75 (N is an integer of at least 3) (see Japanese Patent Laid-Open No. 263932/1985); the ratio of the lens pitch of Fresnel lenses to that of lenticular lenses is defined to fall between 0.1505 and 0.1545 or between 0.1760 and 0.181 (see Japanese Patent Laid-Open No. 149540/1991). The pitch ratio of Fresnel lenses to lenticular lenses claimed in these references of prior patent applications is experimentally defined for reducing moire fringes to a negligible level.
With the increase in the resolution of rays of image light as in typically digital TVs, the lens pitches of the lenticular lens sheet and the Fresnel lens sheet for rear projection screens are being minimized these days. The lens pitch of ordinary lenticular lens sheets heretofore employed in the art generally falls between about 0.7 and 1.2 mm or so, but is being reduced to fall between about 0.1 and 0.52 mm or so with the increase in the resolution of rays of image light. In that situation, however, we, the present inventors have found that, in the combination of a lenticular lens sheet having a reduced lens pitch and a Fresnel lens sheet also having a reduced lens pitch, the ratio of the lens pitch of the Fresnel lens sheet to that of the lenticular lens sheet, which has heretofore been said the best as in the references mentioned above, could not satisfactorily solve the problem of moire fringes.